Conventional alternating-current (AC) electric power generation and distribution systems for marine application employ fixed-speed AC generation. In these systems, prime movers, e.g. diesel generators, gas turbines, etc., operate at fixed speeds and often not at their optimal-efficiency points. The generators typically output medium voltages (MV) to supply propulsion motor drives, while low voltage (LV) loads, such as drilling equipment, are supplied through MV/LV transformers.
FIG. 1 illustrates an example of a conventional marine AC generation and distribution system, in which four prime movers 110 operate at a fixed speed and are each electrically coupled to an AC generator 120. The AC generators 120 are designed to provide a medium-voltage (MV) AC output, on buses MVAC1 and MVAC2. Several loads, indicated as motors 130 in FIG. 1, are driven by the power supplied by AC generators 120, but generally at lower voltages. Accordingly, several of the motors 130 are electrically coupled to the MV AC buses via AC-to-AC converters 140, which provide a variable-voltage output more suitable for the loads. Likewise, others of the motors 130 are driven from low-voltage (LV) AC buses LVAC1 and LVAC2. These LV AC buses are energized by step-down transformers T1 and T2, which, again, provide a lower-voltage output more suitable for the loads.
Where variable-speed prime movers are used, two variable-speed generation schemes are commonly used to decouple the AC-bus frequency from the prime mover speed. One scheme, illustrated in FIG. 2, uses a fully-rated electronic power converter 215 between the generator 120, which is driven by a variable-speed prime mover 210, and the AC bus. A second scheme, illustrated in FIG. 3, uses a doubly-fed induction generator (DFIG) 310 and a fractionally-rated power electronics converter 325. In this second scheme, an interfacing transformer 335 (TDFIG in FIG. 3) is generally used to reduce the voltage rating of the converter and DFIG rotor brushes.